Capacitors are the worst crap around! They shift phases and whatnot. Also, electrolytic caps are prone to leak after a while or even worse, blow up. I try to avoid them in the audio path whenever possible.

Capacitors are the worst crap around! They shift phases and whatnot. Also, electrolytic caps are prone to leak after a while or even worse, blow up. I try to avoid them in the audio path whenever possible.

How bad is the noise of resistors?

It depends on the type of resistor.

Carbon film tend to be more noisy than metal film.
I believe its called Johnson noise.

It greatly depend on the setup.
Some OP are good at higher gain and some are good at lower gain.
Optimum loading and supply also differs.
Your question is hard to answer.
However, with proper setups, everything can sound good.

Is using a high-end op amp for gain stage an acceptable practice in an ultra-high-end amplifier ?

is the practice mostly avoided out of snobbery or are there actual downsides ?

if there are downsides are these subjective or measured ?

also if ther are downsides how can we explain what causes them ?

Yes, IMHO use of quality (not necessarily the most exotic or expensive) op-amps are expected in a high-quality amplifier.

Now IC op-amps, being integrated circuits (ICs) do have limited current and thus power handling capacity, so they typically need to combined with discrete devices (i.e. transistors) for the current and power amplification stage(s) of the signal chain.

There are also discrete op-amp circuits, which can be used to meet a particular set of requirements. Typically IC op-amps combined with a discrete current / power amplification output stage meets most needs.

I think the downside include the lack of mystic (hence pricing premium) for some designers or perhaps they should be called marketers, of the sort who promote and sell more about the audiophile "perception" or snobbery rather than performance.

Other than the current / power handling limitations of IC op-amps, I am not aware of any measurable quality or characteristic that an IC op-amp is inferior compared with discrete components (solid state or electron tube).

One special application where the usage of electron tubes (vacuum tubes / valves) is in electric guitar amplifiers, where the additional distortion of even harmonics may be desirable sonic quality. I believe that even harmonic distortion is considered more pleasuring the human ear, compared to odd harmonics generated from distortion in solid state components.

In terms of transistor matching, I believe (from memory, so don't quote me on it) IC op-amps may be as much as 10-100 times better matching than discrete matched transistors (e.g. LM394) . I believe this is because performance of ICs has demanded continued research in improving the quality of IC manufacturing process so as to turn out higher performance (higher speed digital clocks, faster / wider bandwidth op-amps, higher performance DACs and ADCs) ICs that are a competitive advantage.

Perhaps one downside amongst amateurs is that IC op-amps do require care and modest amount of complexity in circuit design and construction that may not be necessary or the same techniques as found in other circuit designs.